ACPI / battery: Use design-cap for capacity calculations if full-cap is not available

ACPI / battery: Use design-cap for capacity calculations if full-cap is not available

The ThunderSoft TS178 tablet's _BIX implementation reports design_capacity
but not full_charge_capacity.

Before this commit this would cause us to return -ENODEV for the capacity
attribute, which userspace does not like. Specifically upower does this:

        if (sysfs_file_exists (native_path, "capacity")) {
                percentage = sysfs_get_double (native_path, "capacity");

Where the sysfs_get_double() helper returns 0 when we return -ENODEV,
so the battery always reads 0% if we return -ENODEV.

This commit fixes this by using the design-capacity instead of the
full-charge-capacity when the full-charge-capacity is not available.

Fixes: b41901a2cf06 ("ACPI / battery: Do not export energy_full[_design] on devices without full_charge_capacity")
Cc: 4.19+ <stable@vger.kernel.org> # 4.19+
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>